A variety of integrated circuits have been developed for use in applications associated with conventional telephone circuits. These integrated circuits must interface with existing telephone equipment and comply with telephone communication standards. The existing equipment and standards were not necessarily designed for use with integrated circuits and as such it is often difficult to directly implement a telephone function with an integrated circuit.
Integrated circuits offer the possibility of adding many new features to enhance the use of telephones. In many of these features it is required that the circuit be cognizant of the on-hook or off-hook condition of the telephone handset. Although a power terminal is present which makes a transition as the handset is moved on-hook and off-hook, this power signal is subject to intermittent operation and noise such that it cannot serve directly as a reliable indication of the on-hook or off-hook condition of the handset. Therefore, there exists a need for a circuit which can reliably determine the on-hook and off-hook condition of a telephone handset despite the presence of noise and the intermittent operation of telephone circuits.